Radio communication devices include a radio transmitter and receiver coupled to an antenna which emits and receives radio frequency signals to and from a cellular base station. The devices include a microphone for inputting audio signals to the transmitter and a speaker for outputting signals received by the receiver. The fixed and mobile cellular base stations are situated across the countryside, arranged in cells, with each base station in communication with mobile fixed radios within that area of coverage of that base station.
When a new cellular radio system is initially deployed, operators are often interested in maximising the uplink (mobile to base station) and downlink (base station to mobile station) range. Any increase in range means that fewer cells are required to cover a given geographic area, hence reducing the number of base stations and associated infrastructure costs. The range of the link, either the uplink or the downlink, can be controlled principally in two different ways: by adjusting either the power of the transmitter or the gain at the receiver. On the downlink the most obvious way of increasing the range is to increase the power of the base station transmitter. To balance the link the range of the uplink must also be increased by an equivalent amount.
Power radiating from the handset antennas has tended to increase in order to increase the distances between the handsets and base stations or communications satellites with which the handsets use to link up with public fixed telecommunications networks or other handsets. Ultimately, however, the ranges in many systems are uplink limited due to the relatively low transmitted power levels of hand portable mobile stations and because the output power of a transmitter on a mobile is limited to quite a low level to meet national regulations, which vary on a country to country basis. An efficient omnidirectional antenna can improve the uplink.
Fixed wireless access terminals are increasingly being deployed especially in third world and underdeveloped countries, where there is a limited existing wired telephone network. Wireless terminals enable the telephone network to be rapidly expanded by deploying wireless base stations to provide radio coverage using a fixed cellular concept. This can be much faster and less expensive than laying new cables. Existing cellular standards such as IS-54 and GSM can be used as the air interface protocol, except that features such as handover between cells do not have to be implemented because the terminals are at fixed locations.
A fixed wireless terminal is typically intended to be used indoors, in an identical fashion to a conventional wired terminal. Consequently, a user will place the terminal in a location where it is convenient to use. This can present problems if antennas are mounted on the terminal itself, since the location may not necessarily be in a position where the antenna is best placed for transmission and reception of signals. Indeed, the location may be such that the antennas are in a coverage blackspot. These coverage blackspots occur because, inter alia, signals transmitted from a base station, are required to penetrate the users residence. Transmission losses are incurred in such instances when the signal passes into the building, and this is normally at a minimum when the signal propagates through windows or doors, and greatest when the signal has to pass directly through the building walls and floors. This leads to a non-uniform distribution of the signal level inside the users residence, and this is further aggravated by shadowing effects of internal walls and other obstructions. The coverage blackspot may also be due to other external obstacles such as adjacent buildings and the like. In the event that the user does place the terminal in a blackspot, it is possible to connect a remote antenna to the terminal via a coaxial cable, where the remote antenna is perhaps mounted on a wall or window at a good coverage location in the users residence. As an accessory this antenna is required to be low cost, small in size, and versatile in terms of its mounting. Ideally the antenna should be omnidirectional such that the user is not required to orient the antenna in a particular direction, and it should be vertically polarised in common with the signal transmitted by the base station.
In the case of mobile handsets there is also a requirement for high gain antennas, especially ones which are detachable in view of the increasing concern which has arisen over the proximity of handset aerials to the body in general and the brain in particular. These scares have drawn on research carried out by scientists in Australia, America and Sweden, which has suggested that problems such as senile dementia, cancer and asthma might be associated with the use of mobile handsets. Whilst there are conflicting reports suggesting that the handset, in use, excites RF currents in the body and the body actually forms part of the radiator for the handset, public fears have arisen over the use of such handsets, and in particular, repeated prolonged use. The output of a typical handset can be around 0.6 W maximum, of which the user is exposed to about 0.6 mW--a level well below present safety limits suggested by bodies such as the American National Standards Institute (ANSI).
Presently, a number of manufacturers are producing handsets which have patch antennas mounted internally of the handset casing; whilst this may reduce the amount of radiation directed towards the user by reason of the antenna being situated adjacent to a ground plane (although the ground plane will also parasitically radiate), radiating powers need to be increased in order to compensate for the directionality and because the users hand will tend to attenuate the signals--with unknown long term effects. Applicants have a copending patent application which, inter alia, provides a communications handset with a detachable antenna. Nevertheless, the choice of antennas is not simple.
A further antenna structure is detailed in a European Patent Application, EP 0487053A1 in the name of Andrew Corporation. This antenna consists of two conducting strips with alternating wide and narrow sections. The structure is shown in FIG. 4. The structure is essentially a travelling wave structure that appears as an end fed collinear array of dipoles. The radiation pattern is omnidirectional in the azimuth plane, and this structure is used for low cost cellular base station antenna installations; it is not suitable for handsets. The end of the array is either terminated by putting a load across the two ends, or by shorting the two ends across. Taking one section, the narrow conductor looks very much like a microstrip track with the opposite wide section acting as its ground plane. This track then feeds the wide section above it. Each pair of consecutive sections are approximately one half of a wavelength in length. Consequently, it is found that two consecutive wide sections, one on each conducting strip, are in phase and these radiate such that the peak radiation is perpendicular to the axis of the antenna. However, some radiation occurs from the narrow sections as well. A means of suppressing radiation from the narrow sections has been detailed in U.S. Pat. No. 5,339,089. This amounts to adding side walls on to the wide sections, which adds complexity to the structure and therefore cost.